1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a system and method for forming thin films of silicon over a silicon barrier layer by sequential sputter deposition processes.
2. Description of the Related Art
In the manufacture of thin film transistor (TFT) liquid crystal displays (LCDs), the active region of the display consists of an array of pixels, built on a transparent (glass) substrate. The TFTs are in essence optical switches that control the amount of light that is allowed to pass through each pixel before reaching the eye of the viewer(s). TFTs are made using a plurality of typical semiconductor processes, such as deposition, lithography, etching, etc., directly on the glass substrate. Before the TFT fabrication commences, the glass substrate needs to be coated with an appropriate base coat layer, to prevent outdiffusion of impurities, typically present in the glass material, into the TFT layers. The most conventional base coat material is silicon dioxide (SiO2) film. The base coat deposition is usually followed by an annealing (densification) process, to improve the quality of the base coat layer before it comes in contact with subsequently deposited TFT layers.
The need for the annealing step, to improve the material quality, stems from the fact that the water in the barrier base coat layer tends to promote Sixe2x80x94OH bonds. These Sixe2x80x94OH groups lead to the formation of spatially localized (fixed) electrical charges in the base coat that can, in turn, modulate the transistor characteristics, such as threshold voltage and subthreshold slope, when adjacent silicon films are used as transistor active areas. Furthermore impurities on the interface between the base coat layer and the adjoining silicon film may lead to development of interface trap states that will similarly affect the transistor performance. These phenomena are more likely to occur when the adjacent silicon films are thin, such as the silicon films used to form a TFT.
Annealing reduces the amount of water that is trapped in the base coat film. Water in SiO2 films is manifested in two main ways: (1) the number of Sixe2x80x94OH bonds in the film, and (2) the amount of absorbed water in the film upon exposure to ambient. The first source is attributed to the deposition chemistry. That is, the interaction of H, O and Si as the film is formed from precursors such as SiH4 or TEOS. This source can be controlled, to a certain extent, by process optimization, so that the amount of Sixe2x80x94OH bonding configurations is minimized. However, an undesirable amount of Sixe2x80x94OH bonding still occurs. The second source depends upon the physical characteristics of the film, such as porosity, and is difficult to eliminate. Even the densest films will tend to absorb some water upon exposure to the ambient environment after the deposition process.
Another conventional approach used to address the problem of water in the silicon base coat is the formation of a dual base coat layer. When optimized, this process alleviates the need for post-deposition annealing. In this approach, a SiNx/SiO2 stack is deposited. The SiNx layer demonstrates good barrier properties, whereas the SiO2 layer is used to mainly improve the interface quality between the base coat stack and the next TFT layer. This approach, however, requires extra process steps, as two layers must now be deposited. Further, the xe2x80x94OH radicals are not eliminated, merely trapped in the SiO2 film, and the potential still exists for these radicals to create fixed charge groups in the base coat.
It would be advantageous if a TFT, overlying a glass substrate, could be formed with a minimal number of xe2x80x94OH radicals in the silicon dioxide base coat.
It would be advantageous if a TFT silicon dioxide base coat could be formed without the necessity of an annealing step. It would be advantageous if the annealing step could be eliminated without the necessity of a nitride layer interposed between the silicon dioxide layer and a silicon thin film.
It would be advantageous if a TFT silicon dioxide base coat could be deposited with a minimum of hydrogen. It would be advantageous if a thin silicon film could be deposited over the base coat without an intervening process that permits water to be absorbed into the base coat.
The present invention describes processes for the deposition of thin-film materials used in the fabrication of amorphous silicon (a-Si) or polysilicon (polycrystalline silicon) TFTs. The invention involves the sputtering, or physical vapor deposition (PVD) of active amorphous silicon and polysilicon layers, and adjacent insulating base coat layers of silicon dioxide. These adjacent film layers are used in the fabrication of TFTs, which in turn are key elements of different types of LCDs.
The present invention minimizes the formation of Sixe2x80x94OH groups in the oxide barrier layer, significantly improving the quality of the barrier layer. This improvement is accomplished by sputtering the base coat layer and overlying thin silicon film sequentially, to inhibit water absorption. In this manner, the annealing steps, currently employed in conventional TFT fabrication processes to improve the electrical characteristics of the base coat layer, can be eliminated.
Accordingly, in the fabrication of TFTs, a method is provided for forming a thin film of silicon overlying a base coat in a continuous process. The method generally comprises: forming a vacuum seal; sputter depositing a silicon dioxide barrier layer; and, without breaking the vacuum seal, sputter depositing a thin film of amorphous or polycrystalline silicon overlying the barrier layer. More specifically, in the context of forming a TFT for use in an LCD, the method further comprises introducing a glass substrate. Then, sputter depositing a barrier layer includes sputter depositing a silicon dioxide base coat overlying the glass substrate.
The base coat is deposited using either a direct current (DC) magnetron sputtering or a radio frequency (RF) sputtering process. When the DC magnetron process is used, an atmosphere is established that includes argon and oxygen. In some aspects of the invention Ar can be replaced by another inert gas such as Ne and Kr. In some other aspects of the invention additional gases such as He and hydrogen can be also used. The target is either a single-crystal silicon, polycrystalline silicon, or doped silicon material. When an RF sputtering process in used, an atmosphere is established that includes Ar and oxygen, and sometimes helium. A silicon target or a silicon dioxide compound target is used.
The silicon thin film is deposited with a DC magnetron process in an atmosphere of Ar, using a target material of single-crystal silicon, polycrystalline silicon, or doped silicon. In other aspects of the invention a mixture of He and Ar can be used for the deposition of the thin silicon film.
Additional details of the above-described sputtering deposition process and a system for forming a TFT base coat layer and silicon thin film in a continuous sputtering process are provided below.